A gasification facility has been hitherto developed which produces a gasification gas by use of a raw material such as coal, biomass or tire chips as fuel.
FIG. 1 shows an example of a gasification facility under development. The gasification facility comprises a gasification furnace 2 having a fluidized bed 1 of a fluid medium (silica sand, limestone or the like) formed with steam to gasify a charged raw material (coal, biomass, tire chips or the like) for production of a gasification gas and a combustible solid, a combustion furnace 5 into which the combustible solid produced by the gasification furnace 2 is introduced through an introduction pipe 3 along with the fluid medium to form a fluidized bed 4 with a fluidizing gas such as air or oxygen to burn the combustible solid, a medium separator 8 such as a hot cyclone which separates a fluid medium from a combustion exhaust gas introduced from the combustion furnace 5 via an exhaust gas pipe 6 to supply the separated fluid medium via a downcomer 7 to the gasification furnace 2, a medium separator 9 such as a hot cyclone which separates a fluid medium from the gasification gas produced in the gasification furnace 2, and a recovery vessel 10 which recovers the fluid medium separated by the separator 9.
In FIG. 1, reference numeral 11 denotes a distributing plate for uniform blowing of the steam introduced through a bottom of the gasification furnace 2 into the fluidized bed 1; and 12, a distributing plate for uniform blowing of the fluidizing gas introduced through a bottom of the combustion furnace 5 into the fluidized bed 4.
In the gasification facility described above, during normal operation, the fluidized bed 1 is formed with steam in the gasification furnace 2. A raw material such as coal, biomass or tire chips charged into the fluidized bed 1 is gasified by way of steam gasification into the gasification gas and combustible solid. The combustible solid produced in the gasification furnace 2 is introduced along with the fluid medium through the introduction pipe 3 into the combustion furnace 5 having the fluidized bed 4 formed with the fluidizing gas, and is burned. A combustion exhaust gas from the combustion furnace 5 is introduced through the exhaust gas pipe 6 into the medium separator 8 such as the hot cyclone where the fluid medium is separated from the combustion exhaust gas. The separated fluid medium is returned through the downcomer 7 to the gasification furnace 2 and is circulated.
Thus, the fluid medium elevated in temperature by the combustion of the combustible solid in the combustion furnace 5 passes the exhaust gas pipe 6 along with the combustion exhaust gas, is separated by the separator 8 and is supplied through the downcomer 7 to the gasification furnace 2, so that the furnace 2 is kept high-temperatured. A gas produced by and the raw material residual in pyrolysis of the raw material are reacted with steam, so that a water gasification reaction C+H2O=H2+CO and a hydrogen conversion reaction CO+H2O═H2+CO2 occur, resulting in production of a combustible gasification gas or gases such as H2 and CO.
From the gasification gas produced in the gasification furnace 2, the fluid medium is separated by the separator 9 and is recovered by the vessel 10.
When heat is insufficient during the normal operation in the gasification facility, i.e., when sufficient heat is unavailable for the gasification of the raw material in the gasification furnace 2, fuel such as coal, biomass or tire chips same as the raw material supplied to the furnace 2 is supplementarily charged into the combustion furnace 5 for combustion as indicated by imaginary line in FIG. 1 to compensate the insufficient heat. During a circulative preheating operation preparative to the normal operation in the gasification facility, the raw material is not charged into the gasification furnace 2, but the fuel such as coal, biomass or tire chips is supplied for preheating to the combustion furnace 5 and is burned as indicated by the imaginary line in FIG. 1; the fluid medium elevated in temperature by the combustion of the fuel in the combustion furnace 5 passes the exhaust gas pipe 6 along with the combustion exhaust gas, is separated by the separator 8 and is supplied through the downcomer 7 to the gasification furnace 2, whereby the circulative preheating of the gasification facility is performed.
A temperature of the gasification furnace 2 in the gasification facility described above is adapted to be controlled by a circulating amount of the fluid medium which is high-temperatured. If the circulating amount of the fluid medium is increased and decreased, the temperature of the gasification furnace 2 is raised and lowered accordingly. The circulating amount of the fluid medium is usually adapted to be controlled by, for example, adjusting a flow rate of the fluidizing gas introduced through the bottom of the combustion furnace 5.
Generally, a gasification rate or carbon conversion ratio of the raw material charged into the gasification furnace 2 is significantly affected by a temperature and a retention time of the fluid medium in the gasification furnace 2.
Therefore, for example, when on request from a receiver of the gasification gas it is desired to lower the carbon conversion ratio and thus decrease a produced amount of the gasification gas, one method may be decreasing the circulating amount of the fluid medium to lower the temperature of the gasification furnace 2; and when it is desired to raise the carbon conversion ratio and thus increase a produced amount of the gasification gas, one method may be increasing the circulating amount of the fluid medium to raise the temperature of the gasification furnace 2.
Though different from the above-mentioned gasification furnace, a circulating fluidized bed combusting device is disclosed, for example, in Patent Literature 1 in which a return position of a fluid medium to a circulating fluidized bed combustion furnace is changed to maintain stability of a combustion state.
A biomass fuel gasification device is disclosed, for example, in Patent Literature 2 in which a circulating amount of a fluid medium is controlled to keep constant a temperature of a gasification furnace depending on a load.